Hydrocarbons have been sought in North America for over 120 years. After such a long period of exploration, one might expect that little remains to stimulate further search. The fact is that surprisingly few areas have been totally depleted and abandoned; old areas are being continually reworked and new fields are being discovered in the established provinces. The thrust of new exploration is maintained by the presence of large areas underlain by prospective rocks which have received little or no attention in the past and by the search for new pay horizons, usually deeper, in established producing areas. Thus, the areal extent of oil and gas production, as well as the number of producing wells, is continually increasing. It is important to point out, however, that the average production per well has been declining over a long period because of a decrease in the size of the fields being discovered.
The intensity of hydrocarbon exploration has been cyclic. Prosperous times of increased demand and availability of capital have seen high levels of exploration activity, whereas periods of economic downturn have seen a corresponding decline. With each new cycle, economic incentives coupled with the demand for more petroleum have caused corresponding increases in technical capability for exploration.
Technical improvements have motivated and provided for an extension of exploration programs within both established and newer areas. Today, with the tremendous increase in pressure to meet demands for oil and gas, we are seeing advances in technology beyond those of any previous time
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Concepts and Techniques in Oil and Gas Exploration
Exploration for oil and gas has witnessed dramatic changes in its nearly 120 year history. Initially, prospects were located by surface shows or seeps; random drilling was predominant. In the early twentieth century, the anticlinal theory became a dominant element in locating traps. Direct mapping of structures by using magnetic, gravity, and seismic data began in the mid 1920s.
In the last 10 years a true revolution has occurred in the use of seismic data in exploration. To a minor degree, the first use of seismic data to locate reefs and carbonate buildups took place in about 1950, but the main era of more quantitative stratigraphic trap detection began in the late 1960s when direct hydrocarbon detection by the so-called “bright spot” concept was first used in the Gulf Coast Cenozoic offshore. In a very short time since then, an increasingly sophisticated seismic mapping approach has swept through the exploration industry. It is now possible to map seismically subsurface stratigraphy, model stratigraphic analogs, and make comparisons of the recorded data with known analogs in order to “read the subsurface.”
We are now on the verge of an era of synergism in revolution where exploration techniques are being integrated with reservoir delineation and production engineering methods. Synergism, according to Webster, is “the joint action of agents…, which when taken together increase each other's effectiveness.” Recognizing this need as a key to success, several companies have begun to integrate the know-how from geology, petrophysics, and reservoir engineering in developing plays such as the Ozona-Sonora gas play in West Texas.
Predicting accurately all the variables in the subsurface requires the solution of an extremely complex equation. This is because there are so many parameters which cannot be scientifically measured adequately ahead of (or, for that matter, after) the drilling bit. Thus, our exploration efforts are designed to reduce the risk of being wrong in our solution of the “subsurface equation.”
Risk reduction in exploration can be greatly facilitated by bringing all the necessary technical expertise to bear on the problem. The exploration hexagon in Figure 1 illustrates the interrelation among six broad technologies that can be used to minimize the risk of drilling a dry hole. Frequently, there are insufficient data available to make use of all disciplines and often only one or two create a play.
A play is an exploration activity involving a geographically designated and geologically definable volume of rock in which one or more targets for hydrocarbons can be described (see Figure 2).